Advent of Code (Python): copy a 2019/22 p2 solution

Author: IsaacG

advent_of_code/2019/d22.py

@@ -1,38 +1,73 @@
 #!/bin/python
 """Advent of Code, Day 22."""
-# https://www.reddit.com/r/adventofcode/comments/ee0rqi/comment/fbwp0r0/
-
 import collections
 PARSER = str.splitlines
 
 
 def solve(data: list[str], part: int, testing: bool) -> int:
     """Return a card after shuffling."""
-    want_card = 0 if testing else 2019 if part == 1 else 2020
-    deck_size = 10 if testing else 10007 if part == 1 else 119315717514047
+    return (part1 if part == 1 else part2)(data, testing)
+
+
+def part1(data: list[str], testing: bool) -> int:
+    """Track the position of a card through some shuffles."""
+    want_card = 0 if testing else 2019
+    deck_size = 10 if testing else 10007
     # cards = collections.deque(range(deck_size))
     # position = list(cards).index(want_card)
     position = want_card
-    for _ in range(1 if part == 1 else 101741582076661):
-        for cmd in data:
-            if cmd == "deal into new stack":
-                # cards.reverse()
-                position = deck_size - 1 - position
-            elif cmd.startswith("cut"):
-                n = int(cmd.split()[-1])
-                # cards.rotate(-n)
-                position = (position - n) % deck_size
-            elif cmd.startswith("deal with increment"):
-                n = int(cmd.split()[-1])
-                # new_cards = [0] * deck_size
-                # for i, card in enumerate(cards):
-                #     new_cards[(i * n) % deck_size] = card
-                # cards = collections.deque(new_cards)
-                position = (position * n) % deck_size
+    for cmd in data:
+        if cmd == "deal into new stack":
+            # cards.reverse()
+            position = deck_size - 1 - position
+        elif cmd.startswith("cut"):
+            n = int(cmd.split()[-1])
+            # cards.rotate(-n)
+            position = (position - n) % deck_size
+        elif cmd.startswith("deal with increment"):
+            n = int(cmd.split()[-1])
+            # new_cards = [0] * deck_size
+            # for i, card in enumerate(cards):
+            #     new_cards[(i * n) % deck_size] = card
+            # cards = collections.deque(new_cards)
+            position = (position * n) % deck_size
 
     return position
 
 
+def part2(data: list[str], testing: bool) -> int:
+    """Perform many shuffles.
+
+    Code taken from https://github.com/Dementophobia/advent-of-code-2019/blob/master/2019_22_p2.py
+
+    See also,
+    https://www.reddit.com/r/adventofcode/comments/ee0rqi/comment/fbwp0r0/
+    https://www.reddit.com/r/adventofcode/comments/ee0rqi/comment/fbnkaju/
+    https://www.reddit.com/r/adventofcode/comments/ee0rqi/comment/fbnifwk/
+    """
+    del testing
+    position = 2020
+    size = 119315717514047
+    iterations = 101741582076661
+
+    addi, multi = 0, 1
+    for operation in data:
+        if operation == "deal into new stack":
+            multi *= -1
+            addi  += multi
+        elif operation.startswith("cut"):
+            n = int(operation.split()[-1])
+            addi  += n * multi
+        elif operation.startswith("deal with increment"):
+            n = int(operation.split()[-1])
+            multi *= pow(n, -1, size)
+
+    all_multi = pow(multi, iterations, size)
+    all_addi = addi * (1 - pow(multi, iterations, size)) * pow(1 - multi, -1, size)
+
+    return (position * all_multi + all_addi) % size
+
+
 SAMPLE = [
     """\
 deal with increment 7
@@ -60,7 +95,7 @@ def solve(data: list[str], part: int, testing: bool) -> int:
 ]
 TESTS = [
     (1, SAMPLE[0], 0),
-    (1, SAMPLE[1], 3),
-    (1, SAMPLE[2], 6),
-    (1, SAMPLE[3], 9),
+    (1, SAMPLE[1], 1),
+    (1, SAMPLE[2], 2),
+    (1, SAMPLE[3], 7),
 ]